1. Field of the Invention
This invention relates to protective devices for electrical circuits and more particularly to a power vacuum fuse having the fuse elements disposed in a vacuum.
2. Description of the Prior Art
In the usual vacuum type fuse an envelope or housing fabricated from a suitable insulated material such as glass, ceramic, or the like forms a vacuum chamber in which a pair of electrodes are disposed. A fuse link is connected between the electrodes of the vacuum fuse and a continuous current path is established through the fuse. During overload current operation current interruption is initiated by melting of the fuse link. When the fuse link melts an arc is formed across the arcing gap established between the electrodes. The arc vaporizes the link and a portion of the metallic electrode material; and, these particles become ionized to help sustain the arc through which current flows until natural current zero is reached. Some prior art vacuum fuses have been provided with a fusible link throughout the full length of the fuse connected to conductive members at each end of the fuse. Condensation shields are often provided around the fusible link to shield the inside of the insulating housing and to prevent the vaporized fuse link material and electrode material from depositing on the inside of the insulating housing and thus shorting out the fuse. The vaporized fuse link and electrode material sustains an arc until extinction at a current zero. After the current zero point has been reached recovery voltage transients begin building up between the electrodes. If the dielectric strength of the arcing gap, which is formed between electrodes when the fuse length melts, is sufficiently strong to withstand recovery voltage transients, breakdown will not occur, the arc will not reignite and circuit interruption will be complete. If the internal insulating surfaces of the vacuum fuse are not protected, the metallic vapors and particles formed during arcing can condense on the internal surfaces and form a metallic coating. In some instances this coating can form shorting paths and cause the vacuum fuse to fail to interrupt. To protect the insulating suffaces of the vacuum fuse it is customary to provide a metallic shield between insulating surfaces and the arc formed during interruption. Most of the metallic vapor then condenses on the shielding surfaces before reaching the insulating surfaces of the vacuum fuse protecting the insulating envelope or housing of the vacuum fuse, thus preventing a shorting of the vacuum fuse.